A digital versatile disk (hereinafter, referred to as a DVD) is capable of recording digital data at recording density approximately six times that of a compact disk (hereinafter, referred to as a CD), and is known as a ROM or a recordable optical disk having large-volume data such as images formed therein.
In recent years, an optical disk of high recording density having a capacity still larger than the DVD is in demand as a ROM because of improved image quality and the like and also because of an increase in an amount of information to be recorded.
To render recording density of an optical disk higher, it is only necessary, in general, to reduce a spot diameter of a laser beam radiated on the optical disk when recording and reproducing data.
To reduce the spot diameter of a laser beam, it is only necessary to reduce a wavelength of the laser beam and increase numeric apertures (NA) of an objective lens for forming a light spot on the optical disk. In the case of the DVD for instance, it uses a semiconductor laser beam of which wavelength is 660 nm and a light condensing element (objective lens) of NA 0.6. In the case of using a blue laser of which wavelength is 405 nm and an objective lens of NA 0.85 for the sake of rendering the recording density of the optical disk higher, it is possible to obtain a ROM and a recordable optical disk of recording density approximately five times that of the DVD (hereinafter, referred to as a high-density optical disk).
If the NA is increased without changing an outside diameter size of the objective lens so much, a working distance (WD) between the objective lens and the optical disk becomes shorter. To be more precise, in the case of using the blue laser of 405-nm wavelength and the objective lens of NA 0.85, the WD becomes 0.3 mm or so. As wobbling of the optical disk (DVD) is 0.3 mm or so, an optical disk device for recording and reproducing the high-density optical disk is apt to have a clash between the objective lens and the optical disk when a focus servo comes off, when a vibration is exerted while stopping operation, and the like. If an impact scar or a flaw is generated on the objective lens or the optical disk due to the clash, there arises a problem that information recorded on the optical disk cannot be correctly reproduced or information cannot be correctly recorded on the optical disk.
A generally used conventional technique will be described as an instrument which solves the above problem.
FIG. 17 is a top view of a conventional optical disk device included in Japanese Patent Laid-Open No. 2001-319355. FIG. 18 is an X to X sectional view of FIG. 17.
An optical disk device 90 shown in FIGS. 17 and 18 includes an optical head 11, a signal processing circuit 120, a servo control circuit 130, a traverse motor 15, a turntable 16 and a spindle motor 17. For convenience in description, FIG. 17 shows an optical disk 14. The optical head 11 radiates light beams collected on the optical disk 14 and detects reflected light from the optical disk 14 so as to output an electrical signal (light volume signal) according to a detected position and a detected light volume of the reflected light. The signal processing circuit 120 generates and outputs a focus error (FE) signal for indicating a focusing state of a light spot on the optical disk 14, a tracking error (TE) signal for indicating a positional relation between the light spot and tracks of the optical disk 14 and the like according to the light volume signal outputted by the optical head 11. The FE signal and TE signal are generically named servo signals. The servo control circuit 130 generates a driving signal according to the servo signal outputted by the signal processing circuit 120. The driving signal is a signal to be inputted to an actuator coil 117 described later for adjusting a position of an objective lens 115. The light spot on the optical disk 14 is controlled by the driving signal so as not to deviate from a recording layer of the optical disk 14. The spindle motor 17 rotates the optical disk 14 mounted on the turntable 16 at a rotational speed according to recording/reproduction speed. The traverse motor 15 moves the optical head 11 to a desired recording/reproduction position in a radial direction of the optical disk 14.
The optical head 11 includes a semiconductor laser 111, a beam splitter 112, a collimated lens 113, a mirror 114, the objective lens 115, the actuator coil 117, a multi-lens 118, a photodiode 119, an actuator cover 18, an objective lens holder 19 and protection members 13a to 13c. In the optical head 11, the semiconductor laser 111 is a semiconductor laser of a GaN system for performing blue emission, and is a semiconductor laser for radiating blue light beams.
The beam splitter 112 reflects the light beam emitted from the semiconductor laser 111. The collimated lens 113 is a lens for rendering the light beams reflected off the beam splitter 112 as parallel light. The objective lens 115 is a lens for collecting the light beams reflected off the mirror 114 and forming a light spot on the optical disk 14.
The actuator coil 117 moves the objective lens holder 19 having the objective lens 115 attached thereto in a vertical direction or a parallel direction to the optical disk 14 according to a level of an applied driving signal.
The light beams collected on the optical disk 14 transmit through the objective lens 115, mirror 114, collimated lens 113 and beam splitter 112. The multi-lens 118 collects the light beams having transmitted through the beam splitter 112 on the photodiode 119. The photodiode 119 converts incident light beams into the light volume signals.
The photodiode 119 generally has plural light receiving areas. For this reason, the signal processing circuit 120 generates the FE signal and TE signal by utilizing information on the light receiving areas of the photodiode 119 where the light volume signals were detected.
The signal processing circuit 120 reproduces an information signal written to the optical disk 14 based on the light volume signals in a state where the optical head 11 is controlled to form the light spot on the recording layer of the optical disk 14. The optical disk device 90 can record data in the recording layer of the optical disk 14 by rendering optical power of the light beams greater than that on reproduction.
In the case of the conventional optical disk device 90, the optical head 11 is provided with the protection members 13a to 13c projecting further on the optical disk 14 side than an apex of the objective lens 115 installed by surrounding the objective lens 115 in proximity to the objective lens 115 on the objective lens holder 19. The protection members 13a to 13c use an elastomeric resin (a silicon resin or a POM (poly-oxymethylene resin) for instance) or the like as a material that hardly scratches the optical disk 14 even when contacting the optical disk 14.
Therefore, when a focus servo of the optical head 11 comes off, or when a vibration is exerted on the optical disk device 90 while stopping operation, and the like, the protection members 13a to 13c contact the optical disk 14 so that a direct clash between the objective lens 115 and the optical disk 14 can be prevented. To be more specific, the protection members 13a to 13c can prevent flaws from being generated on the objective lens 115 and the optical disk 14.
As for the conventional optical disk device 90, however, there is a possibility that dirt or dust may attach to surfaces of the protection members 13a to 13c or a data recording surface of the optical disk 14. There is a problem that a flaw is generated on the data recording surface of the optical disk 14 if a clash occurs in such a state between the protection members 13a to 13c and the optical disk 14 when the focus servo of the optical head 11 comes off or when a vibration is exerted on the optical disk device 90 while stopping operation. If a flaw is generated on the data recording surface of the optical disk 14, there arises a problem that the optical disk device 90 cannot correctly record data on the optical disk 14 or reproduce data recorded on the optical disk 14.